Cardiovascular disease is a leading cause of morbidity and mortality, particularly in the United States and in Western European countries. Several causative factors are implicated in the development of cardiovascular disease including hereditary predisposition to the disease, gender, lifestyle factors such as smoking and diet, age, hypertension, and hyperlipidemia, including hypercholesterolemia. Several of these factors, particularly hyperlipidemia and hypercholesteremia (high blood cholesterol concentrations) provide a significant risk factor associated with atherosclerosis.
Cholesterol is present in the blood as free and esterified cholesterol within lipoprotein particles, commonly known as chylomicrons, very low density lipoproteins (VLDLs), low density lipoproteins (LDLs), and high density lipoproteins (HDLs). Concentration of total cholesterol in the blood is influenced by (1) absorption of cholesterol from the digestive tract, (2) synthesis of cholesterol from dietary constituents such as carbohydrates, proteins, fats and ethanol, and (3) removal of cholesterol from blood by tissues, especially the liver, and subsequent conversion of the cholesterol to bile acids, steroid hormones, and biliary cholesterol.
Maintenance of blood cholesterol concentrations is influenced by both genetic and environmental factors. Genetic factors include concentration of rate-limiting enzymes in cholesterol biosynthesis, concentration of receptors for low density lipoproteins in the liver, concentration of rate-limiting enzymes for conversion of cholesterols bile acids, rates of synthesis and secretion of lipoproteins and gender of person. Environmental factors influencing the hemostasis of blood cholesterol concentration in humans include dietary composition, incidence of smoking, physical activity, and use of a variety of pharmaceutical agents. Dietary variables include amount and type of fat (saturated and polyunsaturated fatty acids), amount of cholesterol, amount and type of fiber, and perhaps amounts of vitamins such as vitamin C and D and minerals such as calcium.
Epidemiological studies show an inverse correlation of high density lipoprotein (HDL) and apolipoprotein (apo) A-I levels with the occurrence of atherosclerotic events (Wilson et al. (1988) Arteriosclerosis 8: 737–741). Injection of HDL into rabbits fed an atherogenic diet has been shown to inhibit atherosclerotic lesion formation (Badimon et al. (1990) J. Clin. Invest. 85: 1234–1241).
Human apo A-I has been a subject of intense study because of its anti-atherogenic properties. Exchangeable apolipoproteins, including apo A-I, possess lipid-associating domains (Brouillette and Anantharamaiah (1995) Biochim. Biophys. Acta 1256:103–129; Segrest et al. (1974) FEBS Lett. 38:247–253). Apo A-I has been postulated to possess eight tandem repeating 22 mer sequences, most of which have the potential to form class A amphipathic helical structures (Segrest et al. (1974) FEBS Lett. 38:247–253). Characteristics of the class A amphipathic helix include the presence of positively charged residues at the polar-nonpolar interface and negatively charged residues at the center of the polar face (Segrest et al. (1974) FEBS Lett. 38: 247–253; Segrest et al. (1990) Proteins: Structure, Function, and Genetics 8: 103–117). Apo A-I has been shown to strongly associate with phospholipids to form complexes and to promote cholesterol efflux from cholesterol-enriched cells. The delivery and maintenance of serum levels of apo A-I to effectively mitigate one or more symptoms of atherosclerosis has heretofore proven elusive.